Periodic Reporting for period 4 - COHERENCE (Exploiting light coherence in photoacoustic imaging)
Reporting period: 2021-04-01 to 2021-09-30
As photoacoustic imaging relies on detecting ultrasound waves that are very weakly scattered in biological tissue, it can provide acoustic-resolution images of optical absorption non-invasively at large depths (up to several cm), where purely optical techniques have a poor resolution because of multiple scattering. As for conventional purely optical approaches, optical-resolution photoacoustic microscopy can also be performed non-invasively for shallow depth (< 1 mm), or invasively at depth by endoscopic approaches. However, photoacoustic imaging suffers several limitations. For imaging at greater depths, non-invasive photoacoustic imaging in the acoustic-resolution regime is limited by a depth-to-resolution ratio of about 100, because ultrasound attenuation increases with frequency. Optical-resolution photoacoustic endoscopy has very recently been introduced as a complementary approach, but is currently limited in terms of resolution (> 6 μm) and footprint (diameter > 2 mm).
The overall objective of COHERENCE is to break the above limitations and reach diffraction-limited optical-resolution photoacoustic imaging at depth in tissue in vivo. To do so, the core concept of COHERENCE is to use and manipulate coherent light in photoacoustic imaging. Specifically, COHERENCE will develop novel methods based on speckle illumination, wavefront shaping and super-resolution imaging. COHERENCE will result in two prototypes for tissue imaging, an optical-resolution photoacoustic endoscope for minimally-invasive any-depth tissue imaging, and a non-invasive photoacoustic microscope with enhanced depth-to-resolution ratio, up to optical resolution in the multiply-scattered light regime.
In terms of impact for the society, COHERENCE leads to the design of innovative biomedical instrumentations aiming at seeing deeper and better through organs, and therefore participates to the development of new diagnostic tools.
The research to design the two instruments lead to one patent for each instrument, and several publications as research articles in peer-reviewed international journal and oral presentations. At the time of writing this report, 11 articles have been published, 3 are in preparation, and 2 patents have been filed. The corresponding results were presented at conferences through more than 25 oral presentations, given by all members of the team (PhD students, post-doctoral researches, permanent team members and principal investigator).
For the minimally invasive endoscopic device, we discovered a new optical memory effect in multi-mode fiber with square geometry, which paves the way towards calibration-free imaging methods through this kind of very small device. A patent has been filed on the proposed approach.